Surface passivated carbon nanodots prepared by microwave assisted pyrolysis: effect of carboxyl group in precursors on fluorescence properties

Carbon nanodots (CDs) have become one of the hottest topics in the fluorescent nanoparticle family. During the preparation of CDs, two necessary materials are involved, namely, a carbon source and a passivator. Both of them greatly affect the photoluminescence (PL) and quantum yield (QY) of the CDs. We hypothesized that the interaction of carbon sources with passivation agents would make a great difference to the PL properties of CDs. 2-Hydroxyisobutyric acid (HBA), malic acid (MA) and citric acid (CA) with similar chemical structures and different numbers of carboxyl groups for dehydration and carbonization were selected as the carbon sources to investigate the effect of carboxyl groups on the PL properties of CDs. The three kinds of CDs were synthesized by one-step microwave-assisted pyrolysis in the presence of 1,2-ethylenediamine as a passivator. The maximum QYs of the as-prepared CDs from HBA, MA and CA were 11.81%, 24.52% and 56.42%, respectively. In addition, the fluorescence lifetime and nitrogen content increased with the number of carboxyl groups in the carbon source molecule. The different number of carboxyl groups was shown to influence the PL of CDs due to the different ability to conjugate amino groups. Moreover, the results of cell co-culture indicated that the resultant CDs were highly biocompatible and bright, holding great potential for biomedical applications.